The　fluid　flow　and　heat　transfer　characteristics　of　internal　longitudinal　finned　tube　were　investigated　experimentally,　and　were　compared　with　those　of　circular　annulus　tube.　It　was　shown　that　the　internally　longitudinal　finned　tube　had　better　heat　transfer　performance　than　the　circular　annulus　tube　with　increase　of　pressure　drop　simultaneously.　Numerical　results　based　on　the　realized　k-ε　turbulence　model　agreed　well　with　the　experimental　results.　It　was　found　that　there　existed　a　critical　Reynolds　number,　that　is,　when　Reynolds　number　was　less　than　the　critical　value,　the　Nusselt　number　of　internally　longitudinal　finned　tube　was　smaller　than　that　of　circular　annulus　tube　in　laminar　flow.　In　addition,　the　Reynolds　number　for　the　transition　from　laminar　to　turbulent　flow　was　greatly　decreased　due　to　the　existence　of　internal　fins.